This application claims the priority of German patent documents 197 36 138.2, filed Aug. 20, 1997 and 98103365.7, filed Feb. 26, 1998, the disclosures of which are expressly incorporated by reference herein.
The invention relates to a process for determining the condition of a road surface in which light is beamed onto the road surface, and the backscattered light is detected and spectrally analyzed. The light beam contains two light fractions from wavelength ranges with and without significant adsorption by water/ice.
The process according to the invention is used to detect, qualitatively and preferably also quantitatively, the presence of water or ice on the road surface, and can be implemented by a system mounted in a vehicle. For this purpose, generally the backscatter behavior of at least one light wavelength which is not significantly absorbed by water, ice or snow is compared to the backscatter behavior of a wavelength that is absorbed by water, ice or snow. The term "without significant absorption" means that, although there may be a certain low absorption, it is definitely lower than in the case of a wavelength range that exhibits a significant amount of water absorption.
German published patent application DE 41 33 359 A1 describes a similar technique to detect the thickness of a water layer on a road in a zero-contact manner. That is, the light which is beamed onto the road surface contains at least two wavelengths from the short infrared range which are subjected to a water absorption of different intensities. For example, for a sensitive measuring range, a measuring wavelength at 1,450 nm and a comparative wavelength at 1,190 nm with a comparatively lower absorption are selected. On the other hand, for example, for an insensitive measuring range, a measuring wavelength at 1,190 nm and a comparative wavelength at 1,080 nm with an even lower absorption are selected. The thickness of a water layer present on the road surface is then determined, assuming an exponential law of absorption and a "gray" road surface (i.e., one which absorbs all wavelengths at the same fraction).
The assumption of a gray radiator for the road surface is critical. In fact, measurements on various road coverings have demonstrated that. it is valid only to a limited extent. For example, some road coverings exhibit a noticeable rise of their spectral reflection capacity in the presence of increasing wavelengths.
International patent document WO 96/26430 A1 discloses a process in which the spectral backscattering capacity of the respective road surface is taken into account. In a linear approximation, the basic or background influence is determined by performing reference measurements at a wavelength below and a wavelength above the measuring range with a significant absorption by water/ice. An interpolating straight background line (reference approximation curve) is placed through these two reference points. In addition, two backscatter measurements are performed in the interval between the reference points at different wavelengths which carry the water/ice information. Via the subtraction using a straight background line, two corresponding background-cleared measuring values are derived. From the sum, difference and quotient of the measured values, a conclusion is drawn concerning the total existing amount of moisture, the fraction of the already frozen material and the thickness of the water or ice layer.
The process described in WO 96/26430 A1 has the disadvantage that, because of a total of only four supporting points, high precision is unobtainable. On the one hand, it is desirable that the background can be more precisely detected than is possible by means of a linear approximation. On the other hand, the process described in WO 96/26430 A1 does not disclose a starting point from which analysis of a larger number of measuring values is achieved in a suitable manner.
German patent document DE 41 41 446 C1 discloses a process for measuring the thickness of a layer of water, snow or ice on a road surface which is based on run-time measurements.
German published patent application DE 30 23 444 A1 discloses a device for determining the condition of the road, in which infrared radiation having a wavelength at which the reflection capacity of snow is lower than that of the dry road surface, is beamed onto the road surface. The backscattered light is then compared with reference signal levels which correspond to previously classified conditions of the road surface.
German patent document DE 38 41 333 C2 describes a process for monitoring the condition of a road. Here, electromagnetic radiation is beamed onto a tread surface of a vehicle wheel running on the road, and backscattered radiation is analyzed.
German published patent application DE 35 45 366 A1 discloses a measuring device for the visual determination of the thickness of a film of water situated, for example, on a metallic surface, in which light is beamed onto the film of water. The light contains a measuring beam of a wavelength absorbed by the film of water, and a reference beam of a wavelength which is subjected to zero water absorption. The thickness of the film of water is then determined from the ratio of the measuring signal level (representing the backscattered measuring beam ratio) to the reference signal level (representing the backscattered reference beam ratio), multiplied with a temperature correction coefficient. Here, the measuring signal level and the reference signal level are optionally averaged over several measuring operations.
German published patent application DE 40 40 842 A1 describes an infrared microwave sensor system for recognizing the road condition (specifically, for recognizing whether the road is dry, wet or icy). This system contains networks consisting of one or several comparators, or one or several gates or flip-flops for processing the signals of an infrared or microwave receiver. Here, the backscattered intensity of the electromagnetic radiation is analyzed for two different wavelengths or narrow wavelength bands, and the quotient of these two intensities or the detector signal voltages derived therefrom is formed. According to the value of the determined quotient, a conclusion is drawn with respect to whether the road is dry, wet or icy.
One object of the present invention is to provide a process and apparatus for determining the condition of a road surface (particularly with respect to the presence of water, ice and/or snow), with relatively low expenditures in a very reliable and precise manner, as well as independently of the road covering.
This and other objects and advantages are achieved by the process and apparatus according to the invention, in which light beamed onto the road surface contains at least a first light fraction from a first wavelength range which is not significantly absorbed by water or ice including snow, and a second light fraction from a second wavelength range which is significantly absorbed by these materials. Here, the first light fraction contains light of several different wavelengths.
The light backscattered by the road surface is detected and spectrally analyzed. For this purpose, an approximation reference curve is determined based on the spectral data obtained for a first backscatter light fraction which pertains to the several different wavelengths of the first light fraction. This can occur, for example, by determining the approximation reference curve as a parameterized compensating curve over the individual measuring points of these spectral data by using a customary compensating calculation. The approximation reference curve extends along the entire spectral plot of the backscattered light, thus also along the wavelength range of the second light fraction that is characterized by a significant absorption by water/ice. As a result, starting from the measuring points in the wavelength range of the first light fraction (without significant water/ice absorption), the approximation reference curve is interpolated or extrapolated into this wavelength range of the second light fraction.
In this manner, the approximation reference curve forms a reference curve in the wavelength range of the second light fraction. This reference curve reflects, at least approximately, the spectral distribution of the backscattered light in this wavelength range (in the case of a dry road surface). In the case of a wet or icy road, the spectral data of the backscattered light in the second wavelength range deviates noticeably from the approximation reference curve, because of a corresponding absorption of the second light fraction by the water or ice.
This process of the present invention from that disclosed in International Patent Document WO96/26430 A1 in that the second light fraction contains light of several different wavelengths, and the difference between the spectral data of the backscattered light fraction originating from the second light fraction and the corresponding data of the approximation reference curve is determined not only at one or several points, but at as many points as possible as a difference curve. The condition of the road surface can then be determined from the plot of the difference curve, qualitatively and quantitatively in a comparatively precise manner.
In accordance with the present invention, the approximation reference curve can be determined using only the backscattering light ratio which pertains to the wavelength range without significant absorption by water or ice. Alternatively, backscattered light fractions which pertain to the wavelength range with significant absorption by water/ice may be used in addition. In the latter case, information concerning both the plot of the curve (in the case of a dry road) and the absorption-caused downward shift of the spectral plot of the backscattered light may be mixed together in the receiver signals and be separated from one another by a suitable analysis. As a result, the approximation reference curve can be determined, and the desired difference can be determined as the difference between the approximation reference curve and the spectral plot which corresponds to the momentarily scanned road condition.
In general, the process according to the invention uses several detectors with different spectral sensitivities, and/or several light sources with different spectral emissions, each of which makes a separate contribution to the backscattered light. The backscattered light, on the other hand, is determined as the integral of the wavelength via the product of the spectral sensitivity of the respective detector with the respective spectral light intensity. Under a justified assumption that the contributions are mutually independent, each contribution supplies an equation for an equation system. Those parameters of a parameterization of the approximation reference curve as well as the desired difference or the thickness of a water layer are entered as the parameters to be determined. Therefore, when a number n of parameters are defined for the approximation reference curve, a number n+1 of different detectors and/or light sources is required in order to be able to clearly solve the equation system. The corresponding analysis is simplified, for example, if .DELTA.-functions are used for the spectral sensitivity or the spectral light intensity (that is, if the spectral course is analyzed in a punctiform manner for example, point-by-point, interpolation, at different discrete wavelengths).
In an embodiment of the present invention, the first and the second light fractions (which are relevant to the analysis of the backscattered light), are situated in the wavelength range of between approximately 800 nm and approximately 1,100 nm. This has, inter alia, the advantage that reasonably priced silicon photodetectors can be used as backscattered light detectors and no special infrared detectors are required. Another advantage is that, in the selected wavelength range, the penetration depth of the light is sufficiently large to permit a complete penetration of the water layers or ice layers which are typically present on traffic routes. Another advantage is that the wavelength range without significant water/ice absorption is composed of two partial ranges, below and above the second wavelength range. As a result, measuring points for determining the approximation reference curve are present on both sides of the second wavelength range. Subsequently, for the spectral plot of the backscattered light (in the case of a dry road), the spectral plot for the wavelength range with significant water/ice absorption can be precisely interpolated.
In still another embodiment of the present invention, the wavelength range without significant water/ice absorption consists of two partial ranges below and above the second wavelength range. This provides the advantages previously described.
In still another embodiment of the present invention, the integral of the difference curve is determined via the second wave range. The resulting (preferably) standardized integral forms a very precise measurement of the thickness of a water film which may have formed on the road surface.
In yet a further embodiment of the present invention, the center of gravity of the difference curve is determined. It was found that, as the result of this knowledge of the center of gravity's position, a decision can be made as to whether a covering situated on the road surface is a water layer or an ice/snow layer.
In still a further embodiment of the present invention, a conclusion can be drawn from the displacement concerning the temperature of the water layer or ice layer. This is because with a falling temperature of the water layer or ice layer, the position of the center of gravity of the difference curve is displaced toward larger wavelengths. The knowledge of the temperature of the water layer or ice layer, in addition to the knowledge of the layer thickness, supplies valuable information in order to be able to judge whether dangerous slippery ice is present.
In an even further embodiment of the present invention, the driver can be given an early warning which provides an ample amount of time in which to prepare for freezing rain. The possibility of also determining (during the analysis process), the water temperature or ice temperature, offers the advantage that (via the temperature), an advanced warning can be derived if water is present on the road and the temperature is approaching the freezing point.
In still another embodiment of the present invention, an additional discrimination between ice and snow is possible because in the case of snow the difference curve, which is similar to that for ice, has a lower amplitude.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.